1. Field of the Invention
The invention relates to a device for measuring the impedance to fluid flow of a vessel in a living body. The term "vessel," as used herein, refers, but is not limited to blood vessels, biliary tract vessels, urinary tract vessels, lymphatic channels, and various natural and prosthetic conduits that are used to replace or bypass these vessels. In specific, the device produces impedance measurements of the vessel including "direct current resistance," "dynamic impedance" and "capacitance" (these terms will be defined later in this specification).
2. Description of Prior or Contemporary Art
In the practice of clinical and research medicine, it is not infrequent that a practitioner needs to ascertain the impedance to the passage of fluid flow of a vessel structure. The impedance of a vessel can be markedly altered by various natural disease processes.
There have been many prior art devices utilized to determine only flow (not impedance) through blood vessels. Various sensors such as Doppler ultrasound, electromagnetic flow probes, and fiber optic catheters can measure flow. A limiting feature of these methods is the need for pre-existing flow in the vessel, which may not always be present. For example, a proximal obstruction in a blood vessel under study could reduce the distal flow velocity to the point that it is not measurable by these prior art devices. It is this distal vasculature that the physician is most interested in.
Very low flow systems such as the biliary tract cannot have their flow velocities determined by these current methods. The practitioner has to resort to using imaging methods which provide no direct impedance data, and are subject to interpretation. Only the method of impedance determination by direct injection can give objective, reproducible data in all of these cases.
The following published papers by clinical investigators report the use of direct injection to measure DC impedance: Ascer E., Veith F., Morin L.: "Components of Outflow Resistance and their Correlation with Graft Patency in Lower Extremity Arterial Reconstructions," J. Vasc. Surgery, 1:817-28, 1984; and, Parvin S., Evans D., Bell, P.: "Peripheral Resistance Measurement in the Assessment of Severe Peripheral Vascular Disease," Br. J. Surgery, 72: 751-3, 1985. U.S. Pat. No. 4,585,010 issued to Ascer et al., also teaches a manual injection technique of measuring vascular impedance.
In all cases indicated above, the investigators have used either hand injection techniques or mechanical injection by constant flow pumps. In the cases of hand injection techniques, a pressure wave form is generated. By integrating this curve, an average resistance value can be calculated. This method assumes that vessel resistance is a constant--this is an over simplification of the way blood vessels behave. The relationship of pressure and flow is not a linear function. Investigators who have used mechanical constant flow pumps have measured the pressure that is developed at a particular flow setting to calculate DC resistance. The problem of using a constant flow pump is that the resulting vessel pressure may be well outside of the physiologic pressure that this vessel is subject to. The resistance data could actually mislead the clinician. Furthermore, using resistance data obtained in this fashion may not be a valid method of comparing the severity of vessel disease to other patients, since the pressures at which the resistance values are determined are not necessarily within the same range for each patient. Another major problem with this method is the possibility of over pressurizing the vessel during testing and causing possible damage to it.
Capacitance measurements by direct injection and dynamic impedance measurements have not been reported in the medical or patent literature.